U.S. Pat. No. 5,093,637 describes systems in which signals of a QPSK (quaternary phase shift keying) modulation system are compensated for non-linear characteristics of a transmitter amplifier. It is stated in that patent that in Japanese patent disclosure publication number 214843/1986, a memory contains compensating factors which modify the real and imaginary parts of an input signal to effect compensation for the non-linearity of a subsequent transmitting amplifier. By subtracting a portion of the transmitting amplifier signal from the input signal, a correction factor to the data stored in the memory can be obtained, maintaining a compensated amplifier output signal even when characteristics of the amplifier change due to temperature changes. The subject matter of U.S. Pat. No. 5,093,637 relates to a generally similar device, but uses a structure which reduces the memory capacity requirements.
Unfortunately, where multi-level data signals are used, the transmitting amplifier must be a linear type of amplifier. This requires a large precompensation memory. The entire system from transmitter to receiver are for QPSK signals, and utilizes linear transmitters and receivers. The compensation of distortion in a linear system is straight forward.
However digital QPSK signals are desirable to be used for mobile satellite voice and data services, and are being considered for future satellite based personal communication systems. It is clear that power consumption, size and cost are important considerations for the design of mobile and personal communication terminals. In order to reduce the cost, it is desirable to use power efficient non-linear amplifiers such as saturated class-C amplifiers or switching amplifiers, which are typically two to four times more efficient than linear amplifiers, but are inherently highly non-linear. Use of such a power efficient transmitting amplifier directly impacts the size and cost of other terminal Components such as the power supply, batteries and heat sinks. The aforenoted QPSK prior art system, requiring the use of linear or linearized systems, cannot use highly non-linear switching amplifiers or saturated class-C amplifiers.